Leclanche dry cells may be basically defined as including a consumable zinc anode, a cathode mix cake consisting essentially of a mixture of manganese dioxide as the active depolarizer material, a conductive material such as graphite or carbon black and an electrolyte, and a separator medium interposed between the anode and cathode and containing the balance of the electrolyte. The separator medium is commonly a gelled paste which may be composed essentially of electrolyte, corn starch and flour. The electrolyte for the dry cell is an aqueous solution of ammonium chloride and zinc chloride and may contain small amounts of various inhibitors such as mercuric chloride as well known to those skilled in the art.
In the more familiary cylindrical or round type of Leclanche dry cell customarily used in flashlights and the like, the anode usually consists of a zinc cup or can which often serves as the principal container for the cell elements. The cathode mix cake is molded with a central carbon electrode to form a cylindrical bobbin. The bobbin resides in the zinc cup or can and is separated therefrom by the gelatinous electrolyte paste forming the separator medium.
Since these dry cells tend to generate and expel a liquidous product often referred to as "spew" particularly on overdischarge, it is customary to encase the cell proper within an outer container of one sort or another in an effort to accommodate the spew and to prevent the spew which is highly corrosive from coming into contact with the flashlight or other instrument in which the cell is used. Another serious problem resides in the fact that these cells often generate copious quantities of gas on overdischarge which if not properly vented could give rise to the buildup of dangerous internal pressure inside the cell. Thus it is usually the practice to provide some sort of venting means in the structure of the outer cell container. Usually the carbon electrode is porous and serves as an internal venting path within the cell. Unfortunately, the pores of the carbon electrode sometimes are blocked by the spew and consequently either the internal gas pressure builds up to a dangerous level or the spew is forced with the gas into and through the venting means in the outer cell container thereby causing leakage.
Considerable effort has been expended in an attempt to generally improve the performance of the Leclanche dry cell. These efforts have been mainly directed to the improvement of the leakage resistant characteristics of the dry cells and also to the problem of providing maximum service life from the cells at a relatively low cost. Thus, it has been proposed to modify the structure of the outer cell container in various ways in order to make the container stronger and less apt to leak, and by providing superior venting electrodes and gas vents. Various proposals have also been made for improving the service life of the dry cells and probably the most notable has been the contribution of the pasted thin paper or film separator. The use of these thin separators in place of the gelatinous electrolyte paste has made it possible to employ more cathode material for any fixed volume in a particular size cell. One disadvantage of this type of construction is that the cell is more apt to leak "spew" particularly on overdischarge.
It has been recognized for many years now by those skilled in the art that if the formation and movement of the spew could be substantially reduced or eliminated, the dry cell would be much less apt to leak. Despite past research studies concerned with the various aspects of the cell reaction, no practical means has been developed for reducing the amount of spew formed within the dry cell while at the same time maintaining a high level of service. Consequently, resort has been taken mainly to improving the structure of the outer cell container with a view towards accommodating the spew and preventing its leakage from the cell.
In U.S. Pat. No. 3,615,859 it is disclosed that if the gelatinous electrolyte paste separator wall thickness is significantly increased, the formation of spew can be substantially reduced without seriously reducing the service life. It was also disclosed that the use of a thick wall paste separator brings about a distinct change in the character and location of the by-products of the cell reaction while at the same time increasing the electrical efficiency of the cell.
Although the thick wall paste separator type zinc cell exhibits excellent leakage resistance, the cell when subjected to abuse conditions, such as charging, continues to build up gas pressure within the cell until zinc dendritic growth extends through the separator to contact the cathode thereby internally shorting the cell and stopping any further electrolytic generation of H.sub.2. However, due to the thickness of the paste wall separator, there is a possibility that under abuse conditions, the cell may disassemble prior to shorting due to the increased time required for the dendritic growth to extend through the separator.
It is, therefore, an object of the present invention to provide a cell, such as a Leclanche dry cell or a zinc-chloride cell, with an excellent leakage resistance characteristic while at the same time providing a safety means for substantially preventing cell disassembly when subjecting the cell to abuse conditions.
Another object of the present invention is to provide a cell with the benefits of both a thick paste wall separator and a thin paste wall separator.
Another object of the present invention is to provide a cell with a tapered cylindrical bobbin disposed within a cylindrical type anode such that the thickness of the electrolyte paste wall separator between the bobbin and the anode can vary along the longitudinal axis of the cell so as to exhibit the benefits of a thick paste wall separator and a thin paste wall separator.
The foregoing and addittional objects will become more fully apparent from the following description and the accompanying drawings.